How Does Human Being See - DAVIES BLOG

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Tuesday, 7 November 2017

How Does Human Being See

How do we see?
Get this simple phenomena. It is damn simple.
When we look at an object, light rays from each point of the object enter our eyes. As they pass through the cornea, aqueous humor, they are bent(refracted) and brought to focus on the retina to form an image. This image is real, inverted and diminished. Light rays are bent most at the air or cornea surface. The Len makes the final fine bending adjustments needed to focus on the retina.
Note: Reflection of light inside the eyeball is prevented by the pigmented(black) choroid layer.
Each point of light that falls on the retina to form the image stimulates the rods and cones in that region. A pattern of electrical impulses is sent to the brain which interprets it, drawing on past experience. The brain tells us that the object is upright and gives us its real size and distance from the eye. We therefore see the object as it usually is and not as the image is formed on the retina. We see an object most clearly when its image falls on the fovea centralis which is along the optical (principal) axis of the eye lens.
Blind spot.
At the point where the optic nerve leaves the retina, there are no sensory cells and so no information reaches the brain about that part of the image which falls on the blind spot.

Controlling the amount of light.
In bright light, more photoreceptors in the retina are stimulated resulting in an increased flow of nervous impulses to the brain via the optic nerves. This causes the brain to send motor impulses to the muscles in the iris via the parasympathetic nervous system. These impulses induce the circular muscles in the iris to contract and the radial muscles to relax, making the pupils smaller. Less light enters the eyes but the image on the retina remains the same, this reflex action is protective as too much light can damage the retina. In dim light, the brain sends motor impulses via the sympathetic nervous system causing the circular muscles of the iris to relax and the radial muscles to contact. This dilates the pupils allowing more light to enter the eyes.

Our eyes can see nearby, as well as, far away objects clearly. This is possible because we can change the shape of the lens and so alter its focal length appropriately to form a clear image of an object on the retina. This ability is called our eyes power of accommodation.
The lens is made up of many layers of transparent cells which are soft and flexible. It is attached to a ring of muscle in the ciliary body by tough, thread-like suspensory ligaments.

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